A bid to draw a parallel between aliens from outer space and aliens from another country is blunted by a weak plot, forgettable dialogue and sloppy politics.

Writer-director Gareth Edwards' lo-fi sci-fi construct "Monsters" is an attempt to counter noisy, hyper effects-laden alien invasion flicks with something teasing, indie and good for you. Instead, it's like a pendulum swing too far in the other direction.

Set in the wake of a returning U.S. space probe crash in Central America that unleashed extra-terrestrial life forms into most of Mexico, "Monsters" posits an illegal immigration metaphor in which a militarily inclined U.S. quarantines its southern neighbor to keep out the "aliens." That's just background, though, for the movie's real story, a travelogue of sorts in which two stranded Americans — a scruffy photojournalist (Scoot McNairy) and his boss' cute blond daughter (Whitney Able) — make their way back to the U.S. border through a government-declared "infected zone," flirting, pouting, making small talk, cocking an ear at odd jungle noises and falling in love.

It's almost inexplicable that Edwards would set up an overstuffed geopolitical allegory complete with screeching, tentacled behemoths (glimpsed early on in a Humvee-attack prologue) and focus instead on two empty-headed blanks who never seem in any real peril or say anything interesting. (Blame that increasingly overused crutch for "realism": ad-libbed dialogue.)

But he's too in love with his handheld, grainy, on-the-fly atmospherics, captured documentary-style with a skeleton crew in Guatemala, Belize and Mexico, then tweaked with CG touches later — and too rigorous in his withholding approach to showing his otherworldly visitors — or to give any serious attention to keeping us engaged. "Monsters" certainly has its arresting dystopian visuals, including what a toweringly oppressive border wall might look like against a sea of peaceful treetops. But to marry that image with a character calling it "the biggest man-made structure I've ever seen" is a head-slapping act of mood-sabotage.

The sadder irony is that for all the movie's simplistic critique about the way an aggrieved superpower responds to any perceived otherness as a one-dimensional threat, Edwards spends most of "Monsters'" running time in battle-torn Mexico and doesn't consider anybody outside its Anglo protagonists — a whole country's worth of people severely affected by an alien outbreak — worthy of characterization beyond negotiating travel fees, acting as jungle guides or feeding exposition.

Although the vérité aesthetics of "Monsters" will invite comparisons to "Cloverfield" and "District 9," what galls most is the infuriating lack of purpose beyond its backdrop. Despite a tantalizingly setup aftermath, everything else is an afterthought.